The manufacture of tobacco products usually involves treatment of the tobacco being processed with certain additives to enhance the quality and flavor characteristics of the resulting products. In view of the various processing conditions to which the tobacco is subjected, care must be taken so that additives applied to the tobacco are not adversely affected by subsequent processing steps. This is particularly true in the case of flavoring additives which are somewhat volatile because subsequent processing steps involving heat can result in significant losses of the volatile additives from the treated tobacco. Thus, tobacco flavoring additives which are somewhat volatile are generally applied to the tobacco in the form of an alcoholic solution of "top flavors" after the tobacco has already been subjected to the heating and drying conditions which are used during certain processing steps.
In addition to the volatile "top flavor" additives, another category of additives having a relatively low degree of volatility is also customarily applied to tobacco. This category includes materials such as sugars, licorice, cocoa, essential oils, fruit extracts and humectants. These materials are known in the art as "casing" materials and they are applied to the tobacco by dipping or spraying prior to the cutting or shredding operation. While the ingredients used in casing compositions may vary to a considerable degree, the basic components are the sugars and humectants. The sugars serve to import the smoking quality of certain tobaccos such as Burley which are deficient in natural sugars. The humectants impart elasticity to the tobacco and contribute to increased stability of the flavor. It should be noted that water is also used in most casing compositions and can provide improved elasticity or pliability of the tobacco. The elasticity due to water, however, is less permanent than that of humectants because water is removed from the cased tobacco during subsequent processing of the tobacco whereas the less volatile humectants are retained by the tobacco during the heating and drying steps. With regard to the specific humectants used in the treatment of tobacco, glycerine was the original humectant used but other compounds used for this purpose include diethylene glycol, triethylene glycol, propylene glycol, butylene glycol and sorbitol.
Apart from the question of where in the processing sequence, "top flavors" are applied to tobacco, the particular method for applying such additives is of primary concern. The method used must be capable of applying the desired quantities of flavoring material to the tobacco as uniformly as possible. It is not surprising, therefore, that methods and apparatus for applying flavoring materials to tobacco have attracted considerable attention from those skilled in the art. For example, recent improvements in application methods and apparatus are disclosed in U.S. Pat. No. 4,054,145. The complex arrangement described in that patent attests to the difficulties in achieving uniform application of flavoring additives to tobacco.
Perhaps the best known and most widely accepted "top flavor" applied to tobacco is menthol. Menthol is most often applied in the form of a dilute alcoholic solution using a rotating cylinder similar to that shown in U.S. Pat. No. 4,054,145. Alcoholic solutions of menthol have also been applied to tobacco in a pneumatic system as described in U.S. Pat. Nos. 3,548,838 and 3,678,939. In spite of its long history of use, the application of alcoholic solutions of menthol to tobacco is not without its disadvantages. The use of alcohol as a carrier medium for menthol and other flavoring additives is both expensive and hazardous. Since the alcohol is normally not recovered following its removal from the treated tobacco by evaporation, it is necessary to employ venting procedures that will remove most of the alcohol and other volatile materials from the factory area where the tobacco is being treated. Not only do such venting procedures result in losses of menthol and other flavoring additives from the treated tobacco, they also require appropriate steps to prevent the formation of explosive mixtures of alcohol vapors and air. Thus, the alcohol-based flavor application systems incur the cost of the alcohol as well as operation and maintenance of equipment for controlling and removing alcohol vapors released into the factory processing area.
The deficiencies associated with the application of menthol dissolved in alcohol have been recognized by those skilled in the art and have led to the development of methods and apparatus for overcoming these deficiencies. One such method is disclosed in U.S. Pat. No. 3,800,806 (Reissue No. 29,298)which describes a method for applying menthol and other flavorants to tobacco by exposing the tobacco to flavorant vapors under controlled conditions. This method avoids the use of solvents or other carriers and purports to overcome the deficiencies of prior art processes based on the spray application of flavorant solutions. Although the teachings of this patent refer to the variations in flavorant application levels which can occur in prior art processes, no data are provided which would indicate that consistently uniform application levels result from the vaporized flavorant application process disclosed. Indeed, it is stated that the factors affecting deposition of the vaporized flavorant on tobacco include the concentration of flavorant vapor in the process stream, the contact time of flavorant vapor with tobacco, temperature of the air stream in the pneumatic conduit, velocity of air in the conduit, tobacco feed rate and tobacco temperature. In order to maintain the desired concentration of flavorant vapor in the process stream, it is necessary to withdraw samples of the pneumatic stream flowing through the conduit for analysis so that additional flavorant can be injected into the process stream. It is apparent that the number of factors that must be precisely controlled in the process increase processing costs without providing any guarantee that variation in flavorant application levels will not occur due to inadequate control over one or more process parameters. Thus, the method disclosed in U.S. Pat. No. 3,800,806 is not entirely satisfactory.
Another attempt to solve the problem of applying uniform quantities of flavorants such as menthol to tobacco is described in U.S. Pat. No. 3,817,258 and the corresponding U.K. patent specification No. 1,357,057. This patent discloses a method and apparatus for applying a flavorant solution to a shredded tobacco stream just before the tobacco is made into cigarettes. The teachings indicate that menthol dissolved in alcohol is sprayed, preferably in aerosol form, onto a moving layer of tobacco just before the tobacco layer is deposited onto a moving web of cigarette paper on a cigarette making machine. Although the disclosed invention has certain attractive features, it has proven to be somewhat unsatisfactory when used with present-day high speed cigarette making machines. Some of the invention's drawbacks are discussed in U.K. patent application No. 2,030,894 published in April 1980. The net result is that a completely satisfactory solution to the problem of applying uniform levels of flavorants to tobacco in an economical manner continues to elude those skilled in the art.
In this review of background art, it should be noted that a number of references teach the use of solvents other than alcohol for applying menthol and other flavorants to tobacco. For example, U.S. Pat. No. 3,085,581 discloses a process for spraying cigar tobacco with a composition containing menthol, butylene glycol and water. U.S. Pat. No. 4,128,101 (Example IV) discloses the treatment of tobacco with a 5 percent by weight solution of 3-ethoxy-4-hydroxy-benzaldehyde 2,2-dimethylpropanediol acetal in propylene glycol although it is evident from the teachings that solvent systems based on ethyl alcohol are preferred. In German Pat. No. 1,065,767, tobacco is treated with solutions of menthol and certain menthol esters in solvents such as diethylene glycol, 1,3-butylene glycol and isopropyl alcohol. Other references could be cited which teach the use of glycols as solvents for flavoring additives but such teachings are usually found in connection with casing compositions which are based largely on flavorants of low volatility. In spite of the numerous teachings relating to the addition of flavorant additives, there is an apparent lack of recognition among those skilled in the art as to how relatively volatile flavor additives may be applied to tobacco without the use of costly volatile solvents and/or apparatus for incorporating volatile flavorants into tobacco products.